My situation is there are two OS partitions that share the same disk, one is Ubuntu(16.04), of course, and the other is Windows 10. Windows's partition is /dev/sda2(/dev/sda1 is ESP), and Ubuntu's one is /dev/sda3. Ubuntu is installed in Btrfs.
There aren't many spaces in that disk, so I have no choice but to adjust the boundary of two partitions time-to-time, by the usage of the each partition.
Since the header(which probably has the information such as filesystem table, or maybe the bootloader like GRUB has to know the offset of the starting position of the partition) of the Btrfs partition has to be modified, my question is, "Is this still can be bootable with the header that moved?".
I have read many previous questions about the resize of the root partition, but there's no clear answer about the case that made the adjustment from the front of the partition.
If you move the starting point of your boot partition, you need to reinstall grub to add there the information about the new starting point.
It does not matter how the partition is formatted (btrfs or anything else).